The present invention relates generally to AC-to-AC electronic converters and, more particularly, to an AC-to-AC converter and method of operation thereof that provides for soft starting of AC loads, (e.g., AC motors), and other related operations including managing 3-phase AC loads.
AC-AC converters function to convert an AC voltage to another AC voltage. One type of AC-AC converters commonly used in industry are soft starters, which are industrial control devices that make it possible to start AC induction motors smoothly, with a reduced inrush current. Soft starters typically employ silicon controlled rectifiers (SCRs) for reducing voltage applied to the motor and thus reducing the starting inrush current to the motor. While SCRs can be turned on at will, they cannot be turned off in a dynamic and controllable fashion, as SCRs turn off naturally when the current reaches zero. SCRs thus lack the desired capability to provide precise control over current—with more control over the current being desirable in order to reduce harmonics, reduce heat dissipation, and further improve AC motor starting, restarting, and stopping characteristics.
Other devices are available that make dynamic and controllable current interruption possible, such as insulated gate bipolar transistors (IGBTs) for example. IGBTs have been used to start motors in various ways, such as by first rectifying an incoming AC voltage, smoothing the DC voltage with a capacitor, and then inverting it with six IGBTs (three-phase full-bridge DC-to-AC inverter). However, such use of IGBTs is often restricted to complex “motor drives,” and while the use of IGBTs in motor drives works very well, the motor drives that incorporate the IGBTs are typically significantly more expensive than a soft starter. While certain elements in the motor drive can be avoided/eliminated in an attempt to reduce cost, such as avoiding usage of a DC link by using a matrix configuration, such converters use many more devices and are also costly.
Another type of circuit that has been used for providing controllable current interruption is called an “AC chopper.” In an AC chopper, IGBTs are switched on and off at a given, typically high, frequency (kHz or more). The switching is, to a large extent, divorced from the incoming voltage, in terms of timing and phasing. This necessitates the use of capacitors in the AC chopper, in order to provide a path for the energy in the machine inductance when the IGBTs are turned off, as the energy left in the inductance at turn off time can create large voltage spikes, proportional to L*di/dt. These capacitors need to be large, typically commensurate with the machine inductance. As such, in the end, AC choppers require large passive elements such as capacitors, thus incurring a high cost.
It would therefore be desirable to provide a system and method of operation thereof that can start, operate, and stop (brake) AC loads, and particularly electric AC motors, smoothly, conveniently, and with more control than a conventional, SCR-based soft starter, yet is less costly than a drive, a matrix converter, or an AC chopper.